Steel houses are commonly defined as steel panel structure buildings composed of combinations of light-gage shapes of sheet steels not less than 0.4 mm and less than 2.3 mm in thickness and structural panels. When constructing relatively-low multi-floor buildings, such as two- or three-floor building, the platform construction method (the so-called frame-wall construction method), in which the floor of the ground floor is first completed, the wall panels for the ground floor are then mounted on the completed floor, and the floor panel for the upper floor is mounted on the wall panels of the ground panel, have often been used.
This platform construction method has an advantage that heavy machines and scaffolds are unnecessary.
On the other hand, the platform construction method integrates the wall panels of upper and lower stories by using hold-down hardware (sometimes abbreviated as the HD hardware) and long bolts. As, in addition, joist ends of the floor panel are inserted between the wall panels of the upper and lower stories, metal reinforcements which transfer compressive force are inevitably provided to receive the compressive force operating on the joists. The hold-down hardware and metal reinforcements complicate the structure of steel houses. This is a problem in the platform construction method.
The present steel house design method based on the platform construction method is explained by referring to schematic drawings of FIG. 6. As illustrated in FIG. 6, in the platform construction method, the floor of the ground floor (not shown) is first completed, and then the walls 2 of the ground floor is constructed by mounting the wall panels 1 for one story thereon. After the completion of the walls 2 of the ground floor, the floor panel 3 for the upper story is mounted and, then, the walls 4 of the second story are constructed by mounting the wall panels 1 for one story on said floor panel 3.
The wall panel 1 is formed by connecting a structural face plate to a rectangular wall frame which is formed by assembling vertical frame members and top and bottom horizontal frame members to be a rectangle. The floor panel 3 is formed by connecting a floor plate to side and end joists.
In a steel house built as described above by the platform construction method, the walls 2 and 4 of the upper and lower stories are joined together by hold-down hardware 5 and other connection hardware through the floor 3a. 
Japanese Unexamined Patent Publication No. 10-311110 discloses an example of the joined structure described above, as shown in FIG. 7.
In FIG. 7, a wall panel 1 for the upper and lower stories is formed by connecting a structural face plate 13 (hereinafter referred to as the face plate) to a wall frame composed of vertical frame members 10, a top horizontal frame members 11 and a bottom horizontal frame members 12, all of which are made of light-gage channels of sheet steels. The vertical frame members 10 of the upper and lower stories are fastened together by the hold-down hardware 5, as illustrated in FIG. 7.
The floor panel 3 comprising a floor plate 17 mounted on side and end joists of light-gage channels of sheet steels is disposed between the upper end of the wall panel 1 of the lower story and the lower end of the wall panel 1 of the upper story as a partition therebetween. Connection hardware 8 is attached to the floor panel 3.
The connection hardware 8 comprises a cylindrical bolt holder 6 and horizontal flanges 7 fastened at the top and bottom ends thereof, said top and bottom flanges 7 having a bolt insertion hole 7a. The upper and lower wall panels 1 are joined together by connecting a bolt 14 passed through the bolt holder 6 of the connection hardware to the hold-down hardwares 5 attached to the wall panels 1 of the upper and lower stories.
The connection hardware 8 is vertically mounted so as to contact the top and bottom ends of the joists 15 and 16, whereas the bolt 14 is passed along the hold-down hardware 5 of the lower story and through the bolt holder 6 of the connection hardware 8 and the floor plate 17 and the bottom frame member 12 of the wall frame of the upper story, and then fastened by a nut 18 to the hold-down hardware 5 of the upper story. The lower end of the bolt 14 is similarly fastened by a nut 18 to the hold-down hardware 5 of the lower story. Thus, the hold-down hardware 5 joins the wall panels 1 of the upper and lower stories by way of the floor panel 3.
The platform construction method just described requires intricate design that, in turn, makes field work difficult because connection of the wall panel 1 to the floor panel 3 and that of the wall panels 1 of the upper and lower stories require hold-down hardware 5, connection hardware 8 and other metal reinforcements.
If such metal reinforcements are eliminated or reduced in order to avoid an increase in the number of structural members and complicated design, the construction work becomes hazardous. Furthermore, the conventional platform construction method tends to require intricate design because load transfer paths are complicated.
Japanese Unexamined Patent Publication No. 11-140975 discloses a method for improving the platform construction method requiring hold-down hardware. This improving method provides multiple vertical frame studs constituting a wall surface frame so that the studs are expanded throughout the whole stories, and fastens a floor panel on the side of the studs laterally and a wall panel surface member on the surface of the studs vertically.
However, the technology disclosed in Japanese Unexamined Patent Publication No. 11-140975 defies a simple method usable with the platform construction method in which unitized wall panels, which are prepared by fastening a structural surface member to a rectangular wall frame, are joined together, one story after another. The technology disclosed in Japanese Unexamined Patent Publication No. 11-140975 involves a problem that time and trouble in field work increases because wall panel surface members must be attached at the construction site after all vertical frame studs extending to the uppermost story have been joined together.